At low temperatures, polyamides can generally be characterized as having good strength properties (e.g., high tensile strength and toughness, outstanding resistance to chemicals and abrasion, and a low coefficient of friction). Because of these strength properties, polyamides are used in numerous engineering applications that typically do not involve elevated temperatures (i.e., temperatures above 80.degree. C.).
At elevated temperatures (i.e., temperatures above 80.degree. C.), however, polyamides suffer from a reduction in their strength properties. As a consequence of the debilitating effect of temperature, various ingredients have been blended with polyamides in order to yield polyamide compositions that exhibit commercially acceptable strength properties at elevated temperatures. For example, glass products are known to have good strength properties (even at elevated temperatures) and, as a result, have been incorporated into polyamide compositions to improve the strength properties of the resulting compositions which are to be utilized in high temperature environments.
Yet, when modifying a polymer in order to improve one property, e.g., creep resistance at high temperatures, other desirable properties usually suffer. Polyamides and polyamide compositions are no different. That is, although polyamide-glass compositions exhibit improved strength properties at elevated temperatures, they inevitably suffer from poor wear properties. Accordingly, these compositions cannot be utilized in wear applications such as couplings, where good wear properties as well as high strength at elevated temperatures are critical.
Like other strength additives, glass is an extremely "hard" material, and the friction action caused by its presence in a polyamide blend leads to poor wear properties of the resulting blend. In addition, a blend containing a strength additive can also effectuate the undesirable wear of corresponding parts of an apparatus. Hence, the incorporation of a strength additive in a polyamide composition accentuates or compounds the wear problems of any multicomponent apparatus. In this regard, glass as well as other strength additives tend to greatly impair the wear properties of any resulting polyamide composition due to the strength additive's hardness or grit-like characteristics--the very characteristics that seemingly improve the strength of the composition.
Further, many reinforcing type fibers, structures, or agents cannot be commercially utilized in polyamide compositions because of other problems as well. For example, a number of reinforcing agents present major processing complications. In particular, a strength improving additive or component such as an aramid, i.e., an aromatic polyamide fiber, tends to form problematic fiber clumps during processing or blending with polyamides resulting in not only processing problems but non-homogeneous compositions with poor strength properties. Hence, the poor processing characteristics of these reinforcing agents discourages their commercial use in polyamide compositions.
Thus, in addition to the requirements of high strength and low wear properties at elevated temperatures, commercially acceptable polyamide compositions must also exhibit desirable processability characteristics. That is, during the compounding or blending of the molding composition's ingredients, the polyamide must be capable of readily mixing with the other employed ingredients. Otherwise, there is an elevated risk that the final composition will be non-homogeneous. As a consequence of this non-homogeneity, the resulting molded part would suffer from localized weaknesses which would, in turn, render the part commercially unacceptable.
Thus, while the reinforced polyamides known to date may provide improved strength properties at elevated temperatures (versus unreinforced polyamides), they inevitably suffer from poor wear properties and undesirable processability characteristics. Accordingly, the present invention is directed to achieving improvements to the strength, wear, and processability characteristics of polyamide compositions.